An optical document scanning system is used, for example, to input information on a document as image information to a computer. In an optical document scanner, a document image is formed on a solid-state image pick-up device having a photoelectric conversion action, such as a CCD sensor, via the optical scanning system so that the document image is inputted as electrical information to the computer.
Optical scanning systems in use today have a fixed magnification. Nevertheless, optical scanning systems with a variable scanning magnification similar to a photography system used for cameras would be more useful. For example, an optical system having a variable scanning magnification using a variable focal length lens system with more than two movable lenses is described in Japanese Laid Open Patent S54-78150.
In general, each of the lenses forming an optical system is manufactured with some errors (tolerances) in its radius of curvature, lens thickness, distance between lenses, refractive index, and dispersion in relation to desired design values. Hence, in a zoom lens having variable focal lengths, errors in focal length and distance between principal points of each of the lens groups comprising the optical system have occurred. When each lens group is moved along a given path so that the focal length changes with a constant image position, the focal point of the lens groups will vary with respect to the optical axis due to the above-mentioned manufacturing errors. Therefore, it is necessary to adjust the focal point of an optical system to a predetermined position during the manufacturing processes. This is usually called an adjustment of back focus. Adjustment of back focus is described in Japanese Laid Open Patent H1-201633.
However, with a zoom lens adjusted for back focus as shown in Japanese Laid Open Patent H1-201633, the actual focal point deviates from the given design focal point when each lens group is moved to lens positions corresponding to a given design focal point. In an optical scanning system, a conjugate length between a document being scanned and an image surface is a definite distance. Therefore, the deviation of the actual focal point from the given design focal point means that the actual scanning magnification does not match the given design scanning magnification. The conclusion is that a document cannot be scanned at a desired design magnification in a document scanning apparatus which has an optical scanning system adjusted for back focus during the manufacturing processes.